# Frank Solutions for Chapter 5 Heat Class 9 Physics ICSE

### Exercise 5.1

1. What do you mean by heat?

Heat is defined as a form of energy which flows from one point to another on account of temperature difference.

2. Is heat a form of energy?

Yes, heat is a form of energy.

3. Name the SI unit of heat.

Joule is the SI unit of heat.

4. Define one joule of heat.

One calorie is defined as the quantity of heat required to raise the temperature of 1 gram of water through 1°C.

5. What is the relation between joule and calorie?

1 calorie = 4.2 joules.

6. Define temperature and write its SI unit.

Temperature is the degree of hotness or coldness of a body compared to other bodies around it.

SI unit of temperature is Kelvin (K).

7. Why does a piece of ice when touched with hands appear cold?

We feel cold on touching ice because heat flows from our warm hands to cold ice. Due to this flow of heat from hand to ice, the temperature of our hand falls. This is why we feel cold.

8. Distinguish between heat and temperature.

9. Two bodies at different temperatures are placed in contact with each other. State the direction in which heat flows.

Heat flows from a body at a higher temperature to a body of lower temperature.

10. Is it correct to say that heat is the cause of temperature?

Yes, heat is the cause of temperature because temperature of a body rises when the heat flows into the body.

11. What causes a change in the temperature of a body?

Heat changes the temperature of a body due to flow of heat in or out of the given body.

12. Which is more, a calorie or a joule?

Calorie. Because 1 calorie = 4.2 joules.

13. 1 joule = 4.2 calorie

Is this relation correct?

No, the exact relation is as given

1 calorie = 4.2 joules.

14. Two bodies are at same temperature. Do they necessarily contain same amount of heat?

Yes, because the heat flow is only due to temperature difference between the temperature of two bodies.

### Exercise 5.2

1. What do you mean by temperature?

Temperature is the degree of hotness or coldness of a body compared to other bodies around it.

SI unit of temperature is Kelvin (K)

2. What is the normal temperature of human body?

Normal temperature of human body is 37°C.

3. Convert 20°C into Fahrenheit scale.

To convert 20° into°F

t°c /100 = (t°f - 32)/180

⇒ 20/100 = (t°f – 32)/180

⇒ 20 × 180/100 = t°f – 32

⇒ t°f = 36 + 32

⇒ t°f = 68°f

4. What is the upper fixed point on the Celsius scale?

Upper fixed point on the Celsius scale is 100°C.

5. Convert 80 K temperatures on the Celsius scale.

Tk = 80K

⇒ Tc = Tk - 273

⇒ Tc = 80 - 273

⇒ Tc = -153° C

6. Write down the SI unit of latent heat.

SI unit of latent heat is Joule per kg (J/kg).

7. What do you mean by relative humidity?

Relative humidity is defined as the amount of water vapour in the air compared to the amount needed for saturation.

8. Define coefficient of linear expansion. Write its SI unit.

Coefficient of Linear expansion is equal to the change in length of a rod of length 1m when its temperature rises by 1°C. Its SI unit is °c-1.

9. Name the scientist who designed the first thermometer.

Celsius was the scientist who discovered the first thermometer in 1710.

10. What is the principle of calorimetry?

According to principle of calorimetry of mixtures,

Heat gained = Heat lost

11. Write the SI unit of coefficient of cubical expansion.

SI unit of coefficient of cubical expansion is °c".

12. State two uses of a bimetallic strip.

Two uses of bimetallic strip are

1. As thermostat in electric iron
2. As balance wheel in watches

13. Why do telephone wires sag during summer?

Telephone wires sag in summer because due to heat of the sun, the wire expands and increases in length, thus they sag in summer.

14. State the types of thermal expansion. What is the relation between α and γ?

There are three types of thermal expansion

1. Linear expansion
2. Superficial expansion
3. Cubical expansion

Coefficient of linear expansion be α

Coefficient of cubical expansion be γ

γ = 3α

15. Do all substances expand on heating? Give examples.

Not all substances expand on heating. Some examples of substances which do not expand on heating are plastics, polythene and rubber.

16. What is evaporation? Why does it cause cooling?

Evaporation is the phenomenon of a change of a liquid into vapour without raising the temperature. Evaporation needs energy for phase change from liquid to gases. As water evaporates off your skin, it absorbs energy(heat) from the body to make the phase change to gas thus cooling the body.

17. State three factors affecting evaporation.

Factors affecting evaporation are

1. Humidity- more the humidity less is the evaporation
2. Surface area- more the surface area more is the evaporation
3. wind- more the wind more is the evaporation
4. temperature- more the temperature more is the evaporation

18. What are land and sea breezes? Explain with the help of a labelled diagram.

The cold air that blows from land towards sea during night, is called land breeze

The cold air that blows from the sea towards the land during the day is known as the sea breeze. These breezes are the examples of natural convection current.

19. Is conduction possible in gases?

No, the conduction is not possible in gases. Gases are bad conductors.

20. Is conduction possible in vacuum?

No, conduction is not possible in vacuum.

21. What is the velocity of thermal radiations?

The velocity of thermal radiations is equal to the speed of light i.e,.3 x 108 m/s.

22. Why do we wear woolen clothes in winter?

We wear woolen clothes in winter because woolen clothes have tiny pores and air is trapped in these pores and being a bad conductor, the trapped air obstructs the flow of body heat to the surroundings.

23. Why is a newly made quit warmer than an old one?

A newly made quilt is warmer than an old one because the cotton in the old quilt gets compressed and very little air will remain trapped in it, hence heat insulation is quite poor.

24. In cold countries, the water pipes are covered with poor conductors. Why?

In cold countries, water pipes are covered with poor conductors because poor conductor prevents water from freezing and thus prevent these pipes from bursting.

25. Name three devices which are used to detect heat radiations.

Three devices used to detect heat radiations are

1. Blackened bulb thermometer
2. Differential air thermo scope
3. Thermopile

26. What is meant by thermal expansion?

The increase in size of a body on heating is called thermal expansion.

27. What do you mean by linear expansion?

Linear expansion is the increase in length of a solid on heating.

28. Define coefficient of linear expansion.

Coefficient of Linear expansion is equal to the change in length of a rod of length 1m when its temperature rises by 1°c.

29. What is a bimetallic strip?

A bimetallic strip consists of two metal strips- one with high coefficient of expansion and the other with low coefficient of expansion.

30. What is the unit of coefficient of linear expansion?

SI unit of coefficient of linear expansion is °c’.

31. Name the substance which contracts when heated from 0°C to 4°C.

Water is the substance which contracts, when heated from 0°C to 4°C.

32. Define coefficient of volume expansion.

Coefficient of volume expansion is equal to the change in volume of a rod of volume 1m3 when its temperature rises by 1°C.

33. What is the unit of coefficient of volume expansion?

SI unit of coefficient of volume expansion is °C-1.

34. State two uses of bimetallic strip.

Two uses of bimetallic strip are

1. As thermostat in electric iron
2. As balance wheel in watches

35. How can a glass stopper jammed in the neck of a bottle be removed?

We should heat the neck of the bottle because due to heating the neck will expand and loosen the stopper stuck in the neck. In this way, we can easily remove the stopper from the bottle.

36. Why does a thick glass tumbler crack when very hot water is poured in it?

When hot water is poured into a thick glass tumbler, it generally cracks because on pouring hot water in the tumbler the inner surface heats up and expands more as compared to its outer surface. This unequal expansion between the two surfaces causes a strain and the tumbler cracks.

37. Why does a substance expand on heating?

A substance is made up of molecules arranged in a lattice. On heating, the molecules vibrate faster in the lattice and bump into each other harder. So the distance between the molecules increases thus expanding lattice. Thus, the substances expand on heating.

38. What are the different types of thermal expansions?

There are three types of thermal expansion

1. Linear expansion
2. Superficial expansion
3. Cubical expansion

39. A small gap is left between two iron rails of the railway track. Why?

Gaps are left in the railway tracks because the tracks gets heated during the day and as a result they increase in length. If the gaps are not provided, the railway line would buckle outward and may cause derailment.

40. Why are bridges made of steel girders put on rollers?

The beams of the bridges expand maximum during the summer days and contract maximum during the winter nights. If the beams are fixed at both ends on the pillars, they may develop crack due to expansion and contraction. To avoid this, beams are made to rest on rollers on the pillars to provide space for expansion.

41. What is the relation between α and γ?

Let Coefficient of linear expansion be α

Coefficient of cubical expansion be y

The relation between them is

γ = 3 α

42. A copper wire 10 cm long is heated from 20°C to 30°C. Find the increase in length of the wire, if coefficient of linear expansion of copper is 1.7 × 10-5°C-1?

Original length, | = 10 cm = 0.1 m

Rise in temperature, t = 30 - 20 = 10°C

Coefficient of linear expansion, a = 1.7 ×10-5 °C-1

Increase in length = I ×t × a

= 0.1 × 10 × 1.7 × 10-5

= 1.7 × 10-5 m

43. One liter of mercury at 10°C is heated to 30°C. Find the increase in the volume of mercury. The coefficient of cubical expansion of mercury is 1.8 × 10-4 0c-1

Volume of mercury, V = 1 liter

Rise in temperature, t = 30 - 20 = 20°C

Coefficient of volume expansion, y = 1.8 ×10-4°C-1?

Increase in volume = V ×t ×y

= 1 ×20 ×1.8 ×10-4

= 3.6×10-3 liter

44. Why is a ventilator provided in a room?

A ventilator is provided in a room because it helps in removing the hot air from the room and allows the fresh and cold air to come in.

45. Is it possible to heat a liquid or gas from above? Explain your answer.

No, it is not possible to heat a liquid or gas from above because the transfer of heat through convection takes place vertically upwards in liquids and gases.

So if they are heated from above, the liquid or gas at the top will only be heated because most liquids and gases are themselves bad conductor of heat so they cannot conduct heat from top layer to the bottom layer.

46. Explain the following:

(i) Water is heated generally from below.

(ii) Land becomes warmer than water during the day.

(i) Water is heated generally from below because water itself is a bad conductor of heat and the transfer of heat through convection take place vertically upwards.

(ii) Land becomes warmer than water during the day because water has more specific heat capacity so it absorbs the heat and heats up slowly but on the other hand land has less specific heat and it heats up faster than water.

47. State three main characteristics of a thermometric substance.

Main characteristics of thermometric substance are

1. The substance should have high coefficient of expansion so that it is sensitive to the smallest change in temperature
2. The substance should have uniform expansion all over its entire volume
3. The substance should have minimum specific heat so that it absorbs minimum heat from the body under measurement.

48. Name a substance which is an insulator of heat.

Wood is an insulator of heat.

49. (i) Why in cold countries windows have two glass panes with a thin layer of air between them?

(ii) What is the relation between joule and calorie?

(iii) Is it possible to boil water in a thin paper container? Explain with a reason.

(a) In cold countries, windows are provided with two glass panes because in between these two glass panes, a thin layer of air is present: air being a bad conductor obstructs the conduction of heat from the room to outside.

(b) 1 calorie = 4.2 joules

(c) Yes, it is possible to boil water in a thin paper cup because when heated the heat in the paper cup is transferred to the water through convection and paper cup doesn't get sufficient heat to get burnt

50. What is the principle of the thermometer?

Thermometer works on the principle that substances expand on heating and contract on cooling. So we use a thermometric substance which expands and contracts uniformly.

51. State the advantages and disadvantages of mercury and alcohol as thermometric liquids.

Advantages of mercury and alcohol as thermometric liquid:

1. They both are good conductors of heat.
2. They have high coefficient of expansion thus are sensitive to the smallest change in temperature
3. Their freezing points are very low and boiling point is high in case of mercury

Disadvantages f mercury and alcohol as thermometric liquid:

1. Alcohol is transparent and this makes hard to read the thermometer.
2. It does not have uniform expansion.
3. Mercury is less sensitive than alcohol as its coefficient of expansion is less than alcohol.
4. Alcohol is a volatile liquid.

52. What is meant by the lower and upper fixed points of a thermometer?

Lower point of a thermometer is the temperature at which ice starts melting at normal atmospheric pressure i.e. 0°C

Upper point of a thermometer is the temperature at which water just starts boiling at normal atmospheric pressure i.e. 100°C.

53. Draw a neat and labelled diagram of a clinical thermometer.

A clinical thermometer is used to measure the body temperature. The capillary tube has a constriction above the bulb preventing mercury to fall back once it has risen.

54. State one use of each of the following types of thermometers: (i) Laboratory thermometer, (ii) Clinical thermometer, (iii) Six's maximum and minimum thermometer.

(i) Laboratory thermometer is used to measure and observe the temperature of various chemical reactions

(ii) Clinical thermometer is used to measure human body temperature

(iii) Six's maximum and minimum thermometer is used in meteorology and horticulture.

55. Distinguish between a clinical and a laboratory thermometer.

Difference between laboratory and clinical thermometer.

56. Which is the temperature that is common in the Celsius and the Fahrenheit scale?

The temperature that is common in both clinical and Fahrenheit scale is 40°C

Derivation is as follows

Let the temperature be x

C/100 = (F - 32)/180

⇒ x/100 = (x - 32)/180

⇒ x × 180/100 = x - 32

⇒ 9/5x = x - 32

⇒ -4/5x = 32

⇒ x = - 40

57. Convert the following temperatures into Fahrenheit scale:

60°C, 100°, -40°C, 85°C

(a) 60°C

60/100 = (F - 32) /180

⇒ F = 6×18 + 32

= 110° F

(b) 100 ° C

100/100 = (F – 32)/180

F = 180×1 + 32

= 212°F

(c) - 40° C

-40/100 = (F – 32)/180

F = - 4×18 + 32

= 40° F

(d) 85°c

85/100 = (F – 32)/180

F = 85× 18/10 + 32

= 185°F

58. Convert the following temperatures into Celsius scale.

(a) 104 °F

C = (F – 32)× 100/180

C = 72 x 100/180

= 40 °C

(b) 95°F

C = (F – 32)× 100/180

= 63 x 10/18

= 35° C

(c) 113° F

C = (F – 32)× 100/180

= 81 x 10/18

= 45°C

(d) 32°F

C = (F – 32)× 100/180

= 0 x 10/18

= 0°C

59. Draw a neat and labelled diagram of a Six's maximum and minimum thermometer. Explain briefly, the working of this thermometer. Give a use of this thermometer.

Diagram of six’s maximum and minimum thermometer

Six's thermometer is a thermometer which can measure the maximum and minimum temperatures reached over a period of time, usually during a day. It is commonly used wherever a simple way is needed to measure the extremes of temperature at a location, for instance in meteorology and horticulture.

60. Explain the three modes of transfer of heat.

Three modes of heat transfer are

1. Conduction involves the transfer of heat from the hot end to the cold end from particle to particle of the medium.
2. Convection is the transfer of heat from one body to another by actual movement of the particles of the medium
3. Radiation is the transfer of heat from one body to another without the need of an intervening material medium

61. With the help of a diagram, show that water is a bad conductor of heat.

An ice cube is wrapped into a wire mesh and is put at the bottom of the glass test tube. The test tube is then heated near the top of the water. It will be Noticed that though the water at the top starts boiling, the ice at the bottom does not melt. This clearly shows that water is poor conductor of heat.

62. Write a short note on Land Breeze and Sea Breeze.

The cold air that blows from land towards sea during night, is called land breeze

The cold air that blows from the sea towards the land during the day is known as the sea breeze. These breezes are the examples of natural convection current.

63. A wooden knob and a metal latch on a door are both at room temperatures. Explain, why the latch is colder to touch.

A wooden knob and a metal latch are both being at same temperature but it feels colder to touch the latch because metal is a good conductor and as soon as we touch it heat from our hand flows to the latch and we feel cold while on the other hand wood is a bad conductor of heat, heat of our hand does not flow into it therefore it does not feel cold.

64. Draw a neat and labeled diagram of a vacuum flask (thermos flask). Explain, how it minimizes loss of heat by preventing conduction, convection and radiation.

The flask consists of double walled glass container with vacuum between the walls A and B to prevent heat loss due to conduction and convection as vacuum is the excellent insulator .to prevent heat loss by radiation, the inner side of the wall A and outer side of wall B is silvered. It has a narrow mouth which is closed by a non-conducting rubber stopper.

65. A spiral cut from a thick chart paper is pivoted at O and is held slightly above a burning candle (Fig. 1). You will notice that the spiral starts moving. Explain.

The spiral starts moving because due to the flame of the candle the spiral heats up and expands. While expanding, the spiral tries to create space for the extension in length and an outward pull is created which causes the spiral to move.

66. Give scientific reasons for the following:

(i) In winter the human body covered with a blanket keeps warm.

(ii) It is better to use thin blankets to keep the body warm rather than using a single blanket of thickness equal to their combined thickness.

(iii) In winter the birds fluff up their feathers.

(iv) Old quits are less warm than the new ones.

(v) On a hot sunny day, it is advisable to wear light coloured clothes.

(i) In winters, the human body covered with a blanket keeps warm because the blanket has air trapped in it which provide heat insulation to the body from the surroundings and keep us warm.

(ii) It is better to use two thin blankets to keep the body warm rather than using a single blanket of equal thickness because in between the two thin blankets there is more air trapped than in the single blanket of equal thickness so using two thin blankets better heat insulation is provided to the body from the surroundings and keep us warm

(iii) In winter the birds fluff their feathers in order to trap air in their feathers so that the air provides heat insulation to their body from the surroundings and keep them warm and save them from winter.

(iv) Old quilts are less warmer than new ones because the cotton in the old quilt gets compressed and very little air will remain trapped in it, hence heat insulation is quite poor

(v) People wear light coloured clothes in winter because these clothes reflect most of the sun's radiations and absorb only a little of them. Therefore, they keep themselves cool.

67. What is meant by energy flow?

Transformation of Sun's energy in sun-eco system through a food chain is called energy flow.

68. Draw a flow chart to establish that the transfer of sun's energy in a sun-eco system combination in not cyclic.

Heat produced in various conversion processes like production, consumption, etc is not returned to the sun. Thus energy flow of the sun-eco system is not cyclic.

69. State reason why any energy transfer cannot be 100%.

Any energy transfer is not 100% because energy is lost to the surroundings in the form of heat, friction losses during the transfer of energy. Therefore complete energy is not transferred.

70. State briefly the functioning of a biogas power-source.

Bio gas is produced by the action of bacteria on decaying organic matter. The primary source of bio gas in villages is dung of cow, or buffalo. The bio gas is mostly methane which can be used as chief source of light and heat energy.

71. The diagram (Fig. 2) shows two air filled bulbs connected by a U-tube containing mercury kept equidistant from a glassing bulb.

(i) Explain, how will the level X and Y change?

The level Y will rise and the level X will drop because the air in blackened bulb is heated more than clear bulb; hence the air expands and pushes the mercury to the other limb.

72. The following diagrams illustrate three situations involving thermometers which are labeled A, B and C (Fig. 3). In each situation the thermometers indicate different readings.

(i) What do you expect the approximate reading of the thermometer B and C would be? Give a reason for your answer.

(ii) How would the readings of A and B help you in calibrating a thermometer?

(i) Temperature of steam and boiling water is equal so reading of thermometer is same. But due to addition of salt boiling point of water elevates so reading of thermometer would be slightly higher than the boiling point of water i.e. 100°C.

(ii) Due to addition of salt boiling point of water increases so thermometer has to measure temperature for a wider range and thus it helps in calibration of thermometer.

73. A hot metal ball is suspended from a string in a metal box as shown in fig. 4.

(i) What are the ways in which the ball loses heat?

(ii) Which side of the box will be the hottest and which the least hot after sometime? Explain.

(ii) Speed of heat radiation is equal to the speed of light and intensity of heat radiation does net depend upon direction so all side of the box would be equally het as the box is cubical.

74. Fig. 5 shows a metal cylinder, containing boiling water. One half side A, is polished and another half, B is painted black. Two thin metal sheets X and Y are painted black and have one rubber stopper fixed with wax on each sheet. These sheets are equidistant from the boiling water (container A, B) as shown in the diagram. What would you expect to happen after a few minutes? Give reason for your answer.

After a few minutes the rubber stopper fixed to plate y will drop down because since the plate y is on the black side of the cylinder. The black side of the cylinder radiates more heat than the polished side so it heats up the plate Y faster than heating up of plate X by polished surface of the cylinder which melts down the wax holding the stopper and the stopper will drop down.

75. Distinguish between Mercury and Alcohol as a thermometric liquid.

Difference between mercury and alcohol as thermometric substance

76. Water is not used as a thermometric liquid. Why?

Water is not used as a thermometric liquid because It has low coefficient of expansion so it is less sensitive to temperature changes. Moreover, It is transparent thus making it difficult to read the thermometer and water evaporates with time thus producing error and also the freezing and boiling points are also low.

77. How can you increase the sensitivity of a thermometer?

The sensitivity of a thermometer can be increased by using a substance having high coefficient of expansion and uniform expansion so that its expands with the slightest change in temperature.

78. Explain the following:

(i) Why a thick glass tumbler is likely to crack when hot water is poured in it?

(ii) Why does not a Pyrex tumbler crack when hot water is poured in it?

(i) When hot water is poured into a thick glass tumbler, it generally cracks because on pouring hot water in the tumbler the inner surface heats up and expands more as compared to its outer surface. This unequal expansion between the two surfaces causes a strain and the tumbler cracks.

(ii) Pyrex glass tumbler does not crack on adding hot water because Pyrex glass has low coefficient of expansion. It does not expand less when hot water is added to the tumbler.

79. Water at 0°C is heated to 10°C. Sketch a temperature-volume graph to show the behavior on heating.

It can be seen from the graph that the volume of the water decrease from 0°C to 4°C and the volume is minimum at 4°C. After 4°C the volume increases with the increase in temperature.

80. Differentiate between clinical thermometer and laboratory thermometer.

Difference between laboratory and clinical thermometer

81. Study the following diagrams (Fig. 6) and write down your observations.

In the diagram it is dear that the ice will melt faster in the flask than in the test tube because water is bad conductor of heat and the transfer of heat in liquid through convection is from bottom to top

82. The temperature of two bodies A and B differ by 1°C. By how much will it differ on a Fahrenheit scale?

Temperature in °C = 1°C

C/100 = (F - 32)/180

F = 1× 18/10 + 32

= 33.8° F

83. On a faulty thermometer lower fixed point is marked at 10°C and upper fixed point is marked as 130°C. What will be the reading on this thermometer when it is placed in a liquid which is actually at 40°C?

Lower fixed point = 10°C

Upper fixed point =130°C

Range of thermometer = 130°C - 10°C

= 120°C

No of divisions = 100

So least count = 120/100 = 1.2°C

On actual thermometer 40°C would have 40 divisions

So, on this thermometer it would show

= 40× LC = 48°C

84. What is greenhouse effect?

The green house is referred to a glass house. The heat enters the house but cannot escape out, because the glass reflects the heat back to the inside of the house. This makes glass house warmer than the outside environment. This phenomenon is called green house effect.

85. How global warming occurs?

Global warming occurs due to the presence of carbon di oxide, CFCs, methane in the atmosphere. Carbon dioxide acts as a transparent gas to incoming shortwave radiations which the earth re-radiates into space. It, therefore traps the outgoing radiations thus warming lower atmosphere of the earth thereby causing global warming.

86. What are the harmful effects of global warming?

Harmful effects of global warming are

1. The atmospheric temperature of earth would increase thereby making it difficult for a living being to survive
2. It would melt down the polar caps thus increasing the size of the ocean and leading to floods, tsunami, etc.
3. The increase in temperature would affect climate and rainfall thus affecting flora and fauna.
4. Human beings would be vulnerable to diseases as microbes would get warmth to grow.

87. What is the reason behind rising of temperature inside a greenhouse?